The present invention relates to a magnetic resonance imaging (MRI) method using nuclear magnetic resonance phenomenon for obtaining a tomogram, and more particularly to such an imaging method capable of effectively imaging a moving object such as blood vessels, cerebro-spinal fluid (CSF), heart and the like.
Relevant arts to this invention are the following five documents (1) to (5).
(1) IEEE, Trans. on Medical Imaging, MI-1, No. 3, pp. 140-151, 1986 PA0 (2) Japanese Journal of Magnetic Resonance in Medicine, Vol. 8 Supplement-1, p. 236, 1988 PA0 (3) Radiology, Vol. 161, No. 3, pp. 717-720, 1986 PA0 (4) Society of Magnetic Resonance in Medicine (SMRM), Book of Abstracts, Vol. 1, p. 29, p. 446, 1987 PA0 (5) Japanese Patent Laid-open Publication No. 01-131649
A method of magnetic resonance imaging for a moving object (mainly vascular areas) is detailed in the document (1).
According to the principle of selecting vascular areas, a pulse called a flow encoding pulse is used which generates a phase change in accordance with motion. If the flow encoding pulse is present in the flow direction, there is generated a phase change corresponding in amount to the flow rate. Subtraction is then performed between images by a phase sensitive sequence including the flow encoding pulse and a phase insensitive sequence. Since the flow within a vessel is a laminar flow, the vascular area imaged with the phase sensitive sequence has a phase which changes from the center to the periphery of a vessel. Integrated projection data cancel each other so that no signal is obtained from the vascular area. In addition, with the phase insensitive sequence, the phase does not change with motion so that signals are obtained from a laminar vascular area. The stationary area provides signals for both the sequences. The stationary area disappears upon subtraction of images obtained from both sequences, and there appears only the vascular area obtained from subtraction between images. This method is called a subtraction method.
The x-, y- and z-directions in the coordinate system are hereinafter called the readout, phase encoding, and slice directions which are commonly used in this art. FIGS. 4A and 4B shown examples of timing charts of gradient magnetic fields applied in these directions in the subtraction method. FIG. 4A shows the phase insensitive sequence, and FIG. 4B shows the phase sensitive sequence. The sequences shown in FIGS. 4A and 4B are called pulse sequences which are depicted therein as used for one measurement. In the sequences shown, RF and Signal represent the timing of applying a high frequency magnetic filed generated by an RF generator shown in FIG. 3, and the timing of a measured signal. Gs, Gp and Gr represent the timings of gradient magnetic fields respectively in the slice, phase encoding and readout directions.
In the phase sensitive sequence shown in FIG. 4B, first there are applied at the same time a high frequency magnetic field 401 for declining nuclear spins by .alpha. degree and a gradient magnetic field 402 in the slice direction for selective excitation of spins within a slice. Next, there is applied an inverted gradient magnetic filed 403 in the slice direction for alignment of selectively excited spins. Then there is applied a phase encoding pulse 404 to add the information for discriminating the position in the phase encoding direction. At the same time, there is applied an inverted gradient magnetic field 405 in the slice direction for formation of a gradient echo. Thereafter, a gradient magnetic field 406 in the readout direction is applied to measure an echo signal 407. The above operations are repeated at a predetermined repetition frequency (TR) while changing the intensity of the phase encoding pulse 404. In general, the gradient magnetic field (phase encoding pulse 404) in the phase encoding direction is changed 256 times to obtain two-dimensional measurement data necessary for image reproduction. The phase encoding pulse may be changed either starting from a low intensity or from a high intensity.
In contrast, in the case of the phase insensitive sequence shown in FIG. 4A, there are two different points from FIG. 4B. One is that the magnetic fields 402 and 403 serve as the flow encoding magnetic fields in the slice direction in FIG. 4B, whereas in FIG. 4A magnetic fields 408 and 409 are used to make insensitive the phase in the slice direction. The other is that the readout magnetic fields 405 and 406 also serve as the flow encoding magnetic fields in FIG. 4B, whereas in FIG. 4A magnetic fields 410, 411 and 412 are used to make the phase insensitive in the readout direction.
In the angiography using these sequences, there have been proposed the methods of improving the image quality, including (1) a sensitive improvement method in the phase encoding direction, (2) a stationary area signal suppression method of a slice selection type, and (3) a reproduced image phase compensation method.
The description is first directed to the sensitivity improvement method in the phase encoding direction.
Conventionally, as shown in the phase sensitive and insensitive sequences of FIGS. 4A and 4B, any new gradient magnetic field has not been added in the phase encoding direction. The reason for this is that when imaging is carried out at the blood flow rate timings in synchro with a cardiogram, the phase encoding pulses do not disturb the blood phase within a vessel but only generate positional displacement, thus posing no practical problem. However, in the case of the latest high speed imaging wherein imaging is carried out asynchronously, the phase encoding pulses disturb the blood phase so that the blood phase change should be compensated. In the document (3), a compensation method is discussed wherein in addition to the necessary phase encoding pulse, three additional phase compensation pulses are applied.
Next, the stationary area signal suppressing method of the slice selection type will be described.
In the angiography, images projected in the slice direction are picked up. Picked-up images for both the phase insensitive and sensitive sequences have the stationary area in which signals from fine vessels are included. Therefore, most of the dynamic range of measured signals is occupied by the stationary area, thereby posing a problem that essential signals from vascular areas cannot be amplified sufficiently. A means for solving this has been proposed in the document (3) wherein an additional gradient magnetic field (dephasing magnetic field) is applied in the slice direction to disturb the phase at the stationary area and make small those signals therefrom, thereby making relatively large the signals from the vascular area.
There has also been proposed an image quality improvement method not in the imaging level but in the image reproduction level. In the subtraction between two images, since the values of reproduced images have complex numbers, the subtraction should be carried out by using complex numbers. However, the conventional subtraction has used the absolute values of images for the convenience purpose. The reason for this is that since the phase change occurs not only from the blood flow but also from the distortion of the apparatus and from the switching of magnetic fields, the influence of such distortion and switching is required to be eliminated. In the above-described subtraction, pixels having different comlex numbers may have the same absolute value so that a void in the vascular area may occur. A measure for solving this has been proposed in the document (4) at page 29 wherein the phase difference between two images is subjected to a function fitting in consideration of a gentle phase difference therebetween.
As stated above, in the sensitivity improvement method in the phase encoding direction as of the document (2), three additional encoding pulses as well as the essential phase encoding pulse are used. The time from excitation to measurement accordingly becomes substantially longer so that the rapid flow rate such as in the artery cannot be regarded as a constant rate. There arises therefore another problem that a phase change occurs even in the case of the phase insensitive sequence which otherwise does not generate a phase change (1st Problem).
In the stationary area signal suppression method of the slice selection type as of the document (3), an additional dephasing magnetic field generates the first order moment although signals from the stationary area are reduced by the dephasing magnetic field. From this reason, if there is any motion in the slice direction, the phase is disturbed even by the phase insensitive sequence, thereby posing a problem that signals from the vascular area become not likely to be picked up (2nd Problem).
In the reproduced image phase compensation method as described at page 29 of the document (4), not only the function fitting requires an additional operation time, but also the compensation results are dependent on the type of function fitting (3rd Problem)
Further, the last processed vascular images have been displayed merely as a monochrome image so that discriminating between arteries and veins has not been possible (4th Problem).
The conventional methods of improving the image quality of the vascular area have been associated with several problems as above. Image quality could not always be improved by these conventional methods.